Reliable reception of single carrier digital TV (DTV) signals using indoor antennae poses a technical challenge. A single reception site may be served by multiple transmitters, at various locations and at various distances from the receiver. Topology, including terrain and buildings, create multiple paths that each transmitted signal may take from transmitter to receiver, resulting in an impairment phenomenon known as “multipath interference.” The receiver must discriminate among multiple time and phase shifted versions of the same transmitted signal. Reception is usually the best when it is possible to point a receiver's antenna in a direction that receives the strongest signal, with the least multipath interference. In addition, an antenna may have variable gain or other controllable settings that may be used to optimize signal reception.
One solution offered by current technology to solve the problem of indoor reception is the omni-directional antenna. Unfortunately, these antennae tend to exhibit fairly low gain, and tend to exacerbate multipath problems.
A high-gain directional antenna is preferable, but, because signals for different channels come from different directions, it may be desirable that its direction be changed, at least for different TV channels whose transmitters are not co-located. It is unreasonable to expect a consumer, using a remote-control to change channels, to manually change the direction of an antenna just to be able to receive a new channel.
Multiple directional antennae may be combined, with each antenna pointing in a different direction. For example, four antennae may be oriented at 90 degree rotational offsets. The signals from these multiple antennae may be switched, and combined, in order to affect a multi-directional system. For example, if the antenna pointing North and the antenna pointing East are both switched on with equal gain, an effect is created similar to having a single antenna pointing Northeast. The relative gain of adjacent antennae may be controlled to effect a multidirectional antenna. For most reception cases, this is adequate resolution to provide a signal with good signal strength and multipath characteristics, if such a signal is available at all.
The challenge is to develop a system for selecting the proper antenna or combination of antennae, with the proper gain, to give the best TV reception. Another requirement of such a system is that it operate automatically and quickly, so the experience of remote-control channel surfing is not disrupted significantly.
In a multiple-antenna system, the selection of one or more antennae at a given time for reception shall be referred to as an “antenna setting.” One approach may be to try every possible setting, and attempt reception. Any setting that does not give a receivable signal is eliminated immediately. If a receivable signal is found, some quality metric in the demodulator, such as the equalizer signal-to-noise ratio (SNR), may be measured, and the setting giving the best SNR would be selected. While this satisfies the requirement for automatic control, it cannot provide a decision quickly enough for a commercial product.
A much faster method may be to simply measure the signal strength at each setting, and select the setting with the strongest signal. Although this solution works well in many cases, it is not reliable. Empirical evidence shows that the effects of multipath can cause in-phase reflections to add together and appear as a stronger signal than one from the optimal antenna selection. In some cases it is better to direct the antennae toward a weaker signal with better multipath characteristics.
There is clearly a need for a metric value that can indicate signal quality, not just signal strength, in a very short time. The best metric would be one that approximates the equalizer minimum mean square error that would be measured if the receiver were allowed to complete all processes necessary for full signal reception.
Applications on smart antenna remain sparse, and, for the systems of communication, are often focused on applications that improve reception in a mobile environment such as GPS, cellular phone with CDMA technologies, etc.
Smart antenna problems can be divided into issues related to antenna calibration and issues related to an optimization of direction of arrival and/or minimization of undesirable interference by mean of an array of sensors. Optimization of the antenna directivity may or may not be based on an “adaptive” optimization procedure (i.e., an optimization that would track the changes in the channel characteristics). Traditionally, it is desirable for the process to be adaptive when, for example, the antenna has to track a moving target. For a review an adaptive smart antenna systems one may refer to the articles “Adaptive Antenna Systems,” by B. Widrow et al, published in Proceedings of the IEEE, vol. 55, No. 12, December 1967, pp. 2143-2159, and “Adaptive Processing Array Systems,” by W. F. Gabriel, published in Proceedings of the IEEE, vol. 80, No. 1, January 1992, pp. 152-162. The solutions using an adaptive process are often effective in the case of a narrow band signal. For complementary information on this topic one may consult the articles “Two decades of Array Signal Processing Research,” by H. Krim and M. Viberg, published in IEEE Processing Magazine, July 1996, pp. 67-94, and “Spatial and Temporal Communication Theory Using Adaptive Antenna Array,” by R. Kohno published in the IEEE Personal Communications, vol. 5, No. 1, February 1998, pp. 28-35.
Optimization of the directivity of an antenna for a wide band signal is generally a more challenging problem, which can often be merged with a channel equalization problem. This is for example the case when a communication system between multiple links involves multipath. Optimization of reception of a signal source can be associated with the suppression of the intersymbol interference coming from one particular channel of transmission. One may consult the article on channel equalization for Code Division Multiple Access (CDMA) systems “Blind Equalization in Antenna Array CDMA Systems,” by H. Liu and M. Zoltowski, published in IEEE Trans. on Signal Processing, Vol. 45, No. 1, January 1997, pp. 161-172 as an example of smart antenna system using equalization.